1. Field of the Invention
This invention relates to a process for producing and fractionating goal liquefaction products to recover selected compounds for use in the process, and more particularly, to those processes in which a coal-derived solvent is used to dissolve coal in a coal liquefaction zone to yield both distillable and substantially nondistillable coal liquefaction products from which distillable and substantially nondistillable compounds can be separated for use as solvent.
2. Description of the Prior Art
It is known to convert particulate raw coal to coal liquefaction products through the use of distillable coal-derived solvents.
Typically, the particulate raw coal is admixed with the distillable coal-derived solvent to form a slurry which is heated in the presence of hydrogen gas for from about 15 minutes to several hours to a temperature above 850.degree. F. while maintained at a pressure of from 1000 to 5000 psig. to form distillable and substantially nondistillable coal liquefaction products. Recently, it has been found that the liquefaction process can occur quite rapidly at milder reaction conditions than formerly considered necessary. For example, it has been found that over 90 percent of the organic components of a coal in a slurry with a distillable coal-derived solvent can be converted to liquefaction products by heating the slurry for from 1 to 5 minutes at a temperature of from about 800.degree. to 850.degree. F. and a pressure of from about 1200 to 2500 psig. However, it also has been found that at such short reaction times of 1 to 5 minutes a substantial portion of the distillable coal-derived solvent combines with the coal or initial liquefaction products to form a nondistillable product that is unsuitable for reuse as solvent in the liquefaction process. While a portion of the products produced by the liquefaction process comprise the same components as the distillable coal-derived solvent, they are produced in an insufficient amount to replace all of the solvent that is lost through combination with the coal during the liquefaction process. Thus, the coal can be converted under mild reaction conditions but a continuous process would not be possible without the introduction of additional quantities of the distillable coal-derived solvent into each liquefaction process cycle to make up the loss from the previous cycle. Such operation is economically undesirable.
Recently, it also has been found that a portion of the substantially nondistillable coal liquefaction products produced during the liquefaction process comprise compounds that have chemical functionalities typical of the types found in the distillable coal-derived solvents. The potential role of these compounds in a coal liquefaction process has only recently been appreciated.
A theory now has evolved which suggests that these substantially nondistillable coal liquefaction products, which have relatively high boiling point temperatures, also have unique hydrogen transferring abilities similar to those of distillable coal-derived solvents which have a lower boiling temperature and that such products can be used as solvent in the liquefaction process to improve the yield and quality of the process products. The ability to recover these substantially nondistillable compounds from the coal liquefaction products along with the distillable coal-derived solvent provides a means for supplying substantially all of the solvent necessary for continuous short reaction time liquefaction processes.
It is desirable to develop procedures by which the substantially nondistillable coal liquefaction products which exhibit chemical functionalities similar to those of distillable coal-derived solvents can be separated from the coal liquefaction products to permit their use as solvent in coal liquefaction processes.